The invention relates to a transmitter with a quadrature modulator and a power amplifier, which is linearised by what is known as a Cartesian feedback with a quadrature modulator.
EP 0 706 259 A1 discloses a transmitter in which a baseband input signal is applied via two differential amplifiers to a quadrature modulator, which effects a quadrature modulation of the in-phase component and the quadrature-phase component of the complex input signal. The signal is applied to an up-converter, which brings the signal from the baseband up to the transmission frequency. Power amplification takes place in a downstream power amplifier. A feedback loop is provided in order to compensate for the non-linearity of this power amplifier, generally known as a Cartesian feedback. Located in this feedback loop, firstly, is a down-converter which converts the transmission signal uncoupled from the output of the power amplifier back down to the baseband. Located in the baseband is a quadrature demodulator, which separates the feedback signal into a feedback in-phase component and a feedback quadrature-phase component. The feedback in-phase component together with the in-phase component of the input signal is forwarded to a first differential amplifier connected upstream of the quadrature modulator. Likewise, the feedback quadrature-phase component together with the quadrature-phase component of the input signal is applied to a second differential amplifier. The non-linearities of the power amplifier are compensated through the feedback signal as a result.
In order to compensate the DC components of the quadrature modulator, document EP 0 706 259 A1 proposes operating a training sequence during which no input signal is applied to the transmitter. The output signal of the two differential amplifiers is integrated in a respective integrator and applied to a respective sample and hold circuit connected downstream of the integrator. During the training sequence, the sample and hold circuit is in the sampling state and applies a compensation signal to a negative feedback input of the co-operating differential amplifier such that the direct voltage components of the associated arm of the quadrature modulator are compensated. During normal transmission mode, the sample and hold circuit is in the hold state and applies the compensation level determined during the training run to the input of the respective differential amplifier. Document EP 0 706 259 A1 also proposes running another training sequence, during which switches provided on the output of the quadrature modulator are opened, to determine the phase shift for a phase shifter provided between a local oscillator and the quadrature modulator by detecting the output signal of the quadrature modulator in this state with two different input signals.
When using a transmitter which operates on the principle of Cartesian feedback in aeronautical radio communications, particularly with digital aeronautical radio communications operating in TDMA-simplex in accordance with the VDL standard (VHF digital link), a problem arises in that it is very difficult to switch rapidly between transmission mode and reception mode because on switching from transmission mode to reception mode the power amplifier and the local oscillator have to be completely disabled in order to prevent radiation in the receiver. However, the high-frequency feedback loop between the outputs and the compensation inputs of the differential amplifiers necessarily has to be broken in the process. When the power amplifier and the local oscillator are re-enabled as the operating mode is switched from reception to transmission, the feedback loop therefore has to be re-built, which leads to undesirable signal jumps during the switch from reception to transmission. Whilst transmission is interrupted, control of the feedback loop would be adjusted to the positive or negative control end. When re-enabled, the full transmission power would immediately be applied. Document EP 0 706 259 A1 does not suggest any means by which this problem can be eliminated.